Volume Flow Rate vs Velocity

[ShehabGamrah]

I am modelling a lobby with an extract rate of 4.5m3/s and make-up air passing through a 2m by 0.4 m opening. I'd expect the velocity to be 5.625 m/s based on the formula Q= VA. However I am getting velcoities near 7m/s as shown in the image, so how is this possible.

[rmcdermo]

Resolution and vena contra.

[ShehabGamrah]

Thanks, so if I increased the grid size would the velocity equal 5.625m/s based on V=Q/A, and is this the expected velocity in reality as well.

[mcgratta]

Add this to the cross section of the door

&DEVC ID='vflow', QUANTITY='U-VELOCITY', XB=..., SPATIAL_STATISTIC='AREA INTEGRAL' /

This will integrate the u component of velocity (assuming this is that the x direction is perpendicular to the cross section). This should provide you the volume flow through the door.

[ShehabGamrah]

I have done an example where I measured the flow rate at the opening as 3.3 m3/s. Hence, using v=Q/A should equal 3.3/(2x0.4) = 4.18 m/s. However I am getting much higher velocities between 5.5 - 6 m/s. Hence my question if I used a more dense grid would the velocity approach the theoretical calculation. Or is it expected in experimental measurements that the velocity would be higher than the theoretical.

[drjfloyd]

https://en.wikipedia.org/wiki/Vena_contracta